Heap leaching has long been a preferred method of recovering precious metals, such as gold and silver, and base metals, such as copper, from their corresponding ores. Sometimes also referred to as solution mining, heap leaching involves the extraction of soluble metals or salts from an ore by distributing solutions, or lixiviants, during a leaching cycle over an open ore heap piled onto an impervious base. Leach mining may also be performed by vat or agitation leach mining. Typically, dilute aqueous alkaline cyanide is used as a lixiviant for the extraction of gold and silver, and dilute aqueous sulfuric acid is used as a lixiviant for recovery of copper.
The recovery of gold and silver values from low grade ores using oxidative cyanidation is well known. See, e.g., 11 Kirk-Othmer Encyclopedia of Chemical Technology 972-92 (3d ed. 1979). Such recovery methods have been used commercially since the late 1960s. Typically, in heap leaching gold, a dilute aqueous solution of sodium cyanide and lime, having a pH of between about 10.5 and 12.5, is distributed over the top of an ore heap. Ore heaps generally average approximately 100,000 to 500,000 tons in weight and contain ore pieces ranging from less than 1/2 inch to greater than 6 inches in diameter and are piled over an impervious base. Gold is dissolved in an aerated cyanide solution according to the following two-step reaction mechanism: EQU I. 2Au+4CN.sup.- +O.sub.2 +2H.sub.2 O=&gt;2Au(CN).sub.2.sup.- +2OH.sup.- +2H.sub.2 O.sub.2 EQU II. 2Au+4CN.sup.- +H.sub.2 O.sub.2 =&gt;2Au(CN.sub.2.sup.- +2OH.sup.-
J. B. Hiskey, Arizona Bureau of Geology and Mineral Technology Fieldnotes, Vol. 15, No. 4, Winter 1985. The complexed gold is then recovered from the pregnant aqueous lixiviant solution, usually by adsorption onto activated carbon, and the complex is subsequently stripped and converted to elemental gold by electrowinning. The barren cyanide solution is then recirculated to the heap for further leaching, with some replenishment of cyanide if necessary. Leaching of silver is performed analogously, forming the Ag(CN).sub.2.sup.- complex from which elemental silver is usually recovered using zinc dust metal replacement.
Currently, most copper produced by hydrometallurical processing is recovered from the leaching of oxide or secondary sulfide copper ores in heap and dump leach operations. Leaching typically is carried out by sprinkling a dilute solution of sulfuric acid over the top of heaps of broken ore, allowing the acid to trickle through the heaps and dissolving the copper mineralization over a period of several weeks or months. Such hydrometallurgical recovery of copper by leaching primary sulfide deposits is considered difficult and uneconomical due to the refractory nature of the copper mineralization and does not lend itself to sulfuric acid leaching unless oxidative conditions are present during the leach cycle. Recovery of copper values from primary sulfide ore, such as chalcopyrite, typically is limited to conventional pyrometallurgical ore processing by mining, crushing, and ore flotation followed by smelting and electrolytic refining of the copper.
According to Hiskey, supra, compared to conventional milling (i.e., crushing, grinding, and agitation leaching), recovery of gold and silver by heap leaching offers several advantages, among them lower capital and operating costs, shorter start-up times, and fewer environmental risks. Such advantages are, however, offset somewhat by lower metal extractions. Typically, only 60 to 80 percent of available precious metal values can be recovered using state-of-the-art heap leaching techniques. Because many larger ore pieces in heaps are poorly wet, they are poorly extracted. At the opposite extreme, when the larger ore pieces are crushed into smaller pieces to improve extraction, fines are produced that can plug the heap, especially at its bottom, reducing the rate of leachate flow through the heap.
Beneficiation techniques are sometimes employed to increase metal recoveries from ores over those obtained by conventional methods. Beneficiation techniques encompass many and varied processes all with the design to concentrate ore for further processing and extraction. Comminution and agglomeration are the most widely used beneficiation technologies for the recovery of gold and silver. See N. C. Wall, et al. Gold Beneficiation, Mining Magazine (May 1987) (detailing recent developments in beneficiation technology in the extraction of gold). See also A. K. Biswas & W. G. Davenport, Extractive Metallurgy of Copper, Pergamon (3d ed. 1994) (detailing concentration techniques for the extraction of copper).
Oxidative treatments are also sometimes used to increase metal recovery from ores that, because of their particular characteristics, exhibit poor recovery by conventional leaching processes. These so-termed refractory ores may, for example, contain significant concentrations of clays that impede uniform lixiviant percolation or may contain other lixiviant-consuming materials. Oxidizing agents may be used to alter sulfide and carbonaceous gangue mineralizations in such ores, thereby opening leaching channels and/or converting insoluble forms of metals, such as sulfides, into forms more readily soluble in the lixiviants, e.g. oxides and chlorides. Oxidizing gases, including oxygen, ozone, chlorine, and chlorine dioxide, have been used as oxidizing agents, but because of their relatively high vapor pressure, such gaseous oxidizing agents require expensive sealed and pressurized units or volume-limiting, batch-type vessels.
To provide handling convenience and cost-effectiveness, oxidizing reagents are more often applied from aqueous solution. Widely used aqueous oxidizing solutions include hypochlorous acid (generated from sodium hypochlorite), hydrogen peroxide, and nitric acid. These aqueous solutions are, however, less reactive than their gaseous counterparts, often requiring elevated temperatures and significant agitation to complete the ore oxidation process in a reasonably timely manner. Use at elevated temperatures can also lead to oxidizer depletion from hydrolysis that prevents delivery of the oxidizing agent to ore particles in its most reactive and efficient form. As an additional drawback to use of aqueous oxidizing solutions, large volumes of corrosive and pollutant aqueous effluent are created downstream from the leaching process that must be processed and treated, adding additional costs to the overall metal recovery process.
Attempts been made in recent years to provide oxidative treatments comprising saturated aqueous solutions of oxidizing gases. U.S. Pat. Nos. 3,846,124, 4,038,362, and 4,259,107, all to Guay, explore the use of chlorine gas to increase gold recovery from sedimentary gold-bearing ores by slurrying the ore with water and saturating the slurry with chlorine gas prior to cyanidation. In a similar vein, U.S. Pat. No. 4,979,986 (Hill et al.) discloses a method for oxidizing gold-bearing ore by contacting an aqueous slurry of ore with gaseous chlorine or hypochlorite salt and subjecting the slurry to high shear using an impeller. Additionally, U.S. Pat. No. 4,289,532 to Matson et al. describes a process for recovery of gold values from carbonaceous ores by forming an aqueous alkaline ore slurry, subjecting the slurry to oxidation with an oxygen-containing gas, intimately contacting the oxygenated slurry with a source of hypochlorite ions, and subsequently contacting the slurry with a cyanide complexing agent.
Several experimental copper leach-oxidative recovery techniques have been proposed in recent years including: (a) ferric and cupric chloride leaching followed by solvent extraction and electrowinning of copper powder; (b) sulfuric acid-oxygen pressure leaching followed by direct electrowinning of product copper; (c) ammonia-oxygen pressure agitation leaching followed by copper reduction and solvent extraction and electrowinning (Escondida and Arbiter processes); and (d) oxidative roasting of ore followed by sulfuric acid leaching. All of these processes incorporate concentration of the mineralization by crushing and froth flotation of the ore to remove undesired gangue minerals prior to leaching to ensure adequate copper recoveries with minimal reagent waste.